Metabolism of U13C-labeled linoleic acid in lactating women.

Long chain polyunsaturated fatty acids are nutritionally important constituents of breast milk. The origin of these fatty acids in milk has not been clearly identified. We studied the contribution of maternal endogenous conversion of linoleic acid to milk dihomo-gamma-linolenic and arachidonic acids, using stable isotope techniques. Six lactating women ingested 1 mg of [U-13C]linoleic acid/kg body weight in the 2nd, 6th, and 12th week of lactation. Before and at several times during a 5-day period after tracer intake, samples of breath and milk were collected and the volume of daily milk production was recorded. Nutrient intakes were assessed with dietary protocols. The estimated oxidized proportion of the ingested labeled linoleic acid did not differ significantly with duration of lactation (2nd week: 18.9+/-4.5%, 6th week: 24.0+/-3.8%, 12th week: 17.7+/-3.5%, mean +/- SE), nor did transfer into milk as linoleic acid (12.7+/-1.4%, 13.1+/-2.5%, 11.7+/-2.7%, respectively). About 0.2% of the tracer appeared in milk as dihomo-gamma-linolenic acid and 0.02% as arachidonic acid. There were no statistically significant changes with time. We estimate that about 30% of milk linoleic acid is directly transferred from the diet, whereas about 11% of milk dihomo-gamma-linolenic acid and 1.2% of milk arachidonic acid originate from direct endogenous conversion of dietary linoleic acid.